marcgravell.blogspot.co.uk/2014/04/…

> When we did that, we only affected new writes. There was still data using the old format. A lot of data (not just the questions themselves: all tag edits, for example, were stored in this way). But while it remained, our “bulk remove a tag” code had to cope with two formats, which is an unnecessary maintenance overhead. So finally this week we absorbed the pain to do a batched migration of all the old data to the new format. Fairly routine, if a little scary.